This invention relates to a color scanning system which converts a color image into electric signals, and more particularly to a color scanning system which can enhance the quality of a read image.
FIG. 1 is a schematic side view showing a conventional color scanner. Referring to the figure, an original 1 is placed on a platen 2. Arranged under the platen 2 is a fluorescent lamp 3 which is illumination means for illuminating the original 1. In addition, a rod lens array 5 which is focusing means for focusing the color image of the original 1 on an image sensor 4 is arranged near the fluorescent lamp 3.
In the conventional color scanner constructed as stated above, the original 1 put on the platen 2 is illuminated by the fluorescent lamp 3, and the color image of the original 1 is focused into an erect real image with the same size as that of this color image on the image sensor 4 by the rod lens array 5. The fluorescent lamp 3, image sensor 4 and rod lens array 5 are unitarily moved relative to the original 1 and platen 2 in the direction of arrow A. Therefore, the image information items of the original 1 are converted into electric signals sequentially every scanning line.
FIGS. 2 and 3 are a plan view of the image sensor 4, and a plan view showing the layout of color filters in the photodetective area of the image sensor 4, respectively. In FIG. 2, the image sensor 4 is configured of an insulator substrate 41, and a plurality of CCD (charge-coupled device) image sensors 42 which are disposed straight on the insulator substrate 41. In FIG. 3, one picture element 43 is composed of detectors 431-434 which are disposed on one CCD image sensor 42. The detector 431 is a detector which has no color filter (W), and the detectors 432, 433 and 434 are detectors whose front surfaces are formed with the color filters of yellow (Y), green (G) and cyan (C), respectively. Light having fallen on the detectors 431-434 is converted into electric signals, which are externally derived by CCD channels (not shown) disposed on both the sides of the detector array.
Now, there will be explained a method by which output values derived as stated above are converted into (R, G, B) values being ordinary color signals. When Anw, Any, Ang and Anc respectively denote the output values of digital signals obtained by the A/D (analog-to-digital) conversion of the output signals from the detectors 431-434 constituting the n-th picture element 43, the (R, G, B) values are given by the following equation (1): ##EQU1## The matrix of 3 rows and 4 columns used in Eq. (1) is called a "transformation matrix M", which has the following elements by way of example: ##EQU2##
Next, operations in the prior-art color filter array will be explained. Assuming by way of example that yellow light whose magnitude is "2" has entered one picture element 43, the output values Anw, Any, Ang and Anc of the respective detectors 431-434 within the picture element 43 become: EQU Anw=2, Any=2, Ang=1, Anc=1
By substituting these values into Eqs. (1) and (2), the R, G and B values are obtained as follows: EQU R=1, G=1, B=0
and it is found that the color of the light having entered the picture element 43 is yellow.
However, the image of the original 1 detected by the picture element 43 is not limited to a uniform color. A boundary between two different colors could lie in the middle of the picture element 43. How the R, G and B values are on this occasion, will be explained on a case where the boundary of colors lies substantially in the middle of the picture element (at a line L in FIG. 3). Assuming by way of example that white light having a magnitude of "3" has entered the detectors 431 and 433 and that black light having a magnitude of "0" has entered the detectors 432 and 434, the output values Anw, Any, Ang and Anc of the respective detectors 431-434 become: EQU Anw=3, Any=0, Ang=1, Anc=0
In accordance with Eqs. (1) and (2), the R, G and B values are calculated as follows: EQU Rn=3, Gn=1, Bn=3
These values indicate a color of magenta group, and it is understood that noise develops in which magenta mixes on the boundary line between white and black.
As stated above, the color scanning system in the prior art has had the problem that, in such a case where the boundary part of colors on the original 1 comes to lie in the middle of one picture element 43 in the operation of reading a color image, the photoelectric conversion outputs of the picture element 43 exhibit a color quite different from the colors of the original 1, to incur noise in the contour part of a read image.
For the purpose of reducing the aforementioned noise, it is sometimes practised to make the start times of the storage periods of the CCD image sensor different and to provide buffer memory circuits which compensate the attendant shifts of the output signals. This measure will now be explained with reference to FIG. 4 thru FIG. 9.
In FIG. 4, which is a more detailed layout plan corresponding to FIG. 3, numerals 141 and 142 designate transfer gates through which charges generated by input light in the detectors 431, 432 and the detectors 433, 434 are respectively transferred to CCD channels 151 and 152.
In addition, FIG. 5 is a timing chart showing the conventional operation of the CCD image sensor 42.
The CCD image sensor 42 operates as follows: When the transfer gate 141 is in its "off" state, the light entering the detectors 431, 432 is converted into the charges, which are stored in these detectors. Subsequently, when the transfer gate 141 falls into its "on" state, the stored charges are transferred to the CCD channel 151. Potential wells (not shown) corresponding to the individual detectors 431, 432 exist in the CCD channel 151. By impressing two-phase clock pulses .phi.1 and .phi.2 on the CCD channel 151, the charges transferred thereto from the detectors are successively shifted to the adjacent potential wells until they are derived as analog signals out of the CCD image sensor 42 by a floating diffusion amplifier (not shown) provided at the final stage of the CCD channel 151. In actuality, as illustrated in the timing chart of FIG. 5, transfer gate pulses .phi.T are input at a period of a time TS (FIG. 5, (a)) while the two-phase clock pulses .phi.1 and .phi.2 (FIG. 5, (b) and (c)) are being continuously impressed. In this case, the charges transferred to the CCD channel 151 by the second transfer gate pulse .phi.T are equal to the charges stored in the detectors 431, 432 during the preceding time TS. Accordingly, signals whose magnitudes are proportional to the quantities of the light having entered the detector 431 formed with no color filter and the detector 432 formed with the yellow color filter can be alternately derived as time series signals from the floating diffusion amplifier at the final stage of the CCD channel 151. The operations of the detectors 433, 434, transfer gate 142 and CCD channel 152 on the other side are similar to the operations stated above.
In a prior-art example, the method of preventing the noise at the boundary part of the read image proceeds as stated below:
Symbols .phi.1 and .phi.2 shown at (a) and (b) in FIG. 6 denote two-phase clock pulses which are normally and continuously impressed on the CCD channels 151 and 152 in FIG. 4. Symbols .phi.T.sub.GA and .phi.T.sub.GB shown at (c) and (d) in FIG. 6 denote transfer gate pulses which are impressed on the transfer gates 142 and 141, respectively. The periods TSA and TSB of the respective transfer gate pulses .phi.T.sub.GA and .phi.T.sub.GB become storage times. When the lower part of FIG. 4 with respect to a center line L' is defined as channel-A and the upper part as channel-B, it can be said that the storage time of the channel-A is the time TSA, while the storage time of the channel-B is the time TSB. In this prior-art example, the periods TSA and TSB are equal, but the start times of the storage times of the individual channels differ by a time interval TD. As shown at (e) and (f) in FIG. 6, accordingly, output signals D.sub.A and D.sub.B from the channel-A and channel-B are respectively delivered in the order of signals A1g, A1c, A2g, A2c, . . . and in the order of signals A1w, A1y, A2w, A2y, . . . immediately after the impressions of the transfer gate pulses .phi.T.sub.GA and .phi.T.sub.GB.
Next, the operation of the color boundary part will be explained. FIG. 7 shows those positions (in a vertical line scan direction) of the original 1 (in FIG. 1) at which the detectors of the channel-A and channel-B in the color scanner exist with the lapse of time. Letter P in FIG. 7 designates a vertical line scan pitch. It is assumed that, at a point of time t.sub.0 indicated in FIG. 7, the detectors 433, 434 of the channel-A lie at a position y0, while the detectors 431, 432 of the channel-B lie at a position y0-(P/2). In the color scanner, the individual detectors 431-434 are moved relative to the original 1. A line S1 in FIG. 7 indicates the moved states of the detectors 433, 434 of the channel-A, and a line S2 the moved states of the detectors 431, 432 of the channel-B. When the storage time is started at the point of time t.sub.0, light which enters the detectors 433, 434 of the channel-A during the storage time TSA is the reflected light of a part from the position y0 to a position y0+P on the surface of the original 1. The detectors 431, 432 of the B-channel photoelectrically convert the image of a part from the position y0-(P/2) to a position y0+(P/2) on the surface of the original 1. This operation has been the major cause of the generation of the noise at the color boundary part. Therefore, the start time of the storage time TSB of the channel-B is set at t.sub.0 +(TSA/2). Thus, the part from the position y0 to the position y0+P on the surface of the original 1 can be photoelectrically converted also for the channel-B as seen from FIG. 7. Accordingly, even when the color boundary part of the original 1 lies between the positions y0 and y0+P, merely the neutral tint of the colors on both the sides of the boundary appears, and no noise can be formed.
The output signals of the individual channels delivered from the CCD image sensor 42 in this way are process as stated below: FIG. 8 is a timing chart showing the storage times TSA, TSB of the respective channels and the timings of the deliveries of the output signals D.sub.A, D.sub.B.
The respective output signals D.sub.A, D.sub.B begin to be successively delivered immediately after the corresponding storage times have ended. In FIG. 8, the delivery intervals of the output signals D.sub.A, D.sub.B are indicated by hatched lines. The picture element signals of the picture elements G, C of the output signal D.sub.A and those of the picture elements W, Y of the output signal D.sub.B shift by a time interval T.sub.D. In converting the output signals into the (R, G, B) values on the basis of Eq. (1) stated before, therefore, it is necessitated that the output signal D.sub.A is delayed for the time interval T.sub.D into a signal D.sub.A ' as illustrated in FIG. 8, whereupon Eq. (1) is calculated using the signals D.sub.A ' and D.sub.B.
FIG. 9 is a fundamental block diagram of a color conversion circuit which converts the picture element signals G, C, W and Y into the R, G and B values. The output signal D.sub.A delivered from the CCD channel 151 as shown at (d) in FIG. 5 has only its output signal components sampled and held by a sample-and-hold circuit 171 in FIG. 9. Thereafter, the time series signals G and C are separated into individual signals G and C by a demultiplex circuit 181. Numerals 191 and 192 designate the buffer memory circuits which serve to delay the respective individual signals G and C for the time interval T.sub.D. Symbols G' and C' in the figure denote image signals delayed for the time interval T.sub.D with respect to the respective signals G and C.
The output signal DB delivered from the CCD channel 152 as shown at (e) in FIG. 5 is similarly separated into individual signals W and Y through a sample-and-hold circuit 172 and a demultiplex circuit 182. The individual signals G', C', W and Y sampled and held by the above processing are input to a matrix calculation circuit 110 for calculating Eq. (1) mentioned before, thereby to be converted into the R, G and B values.
As thus far explained, the prior-art signal processing method is such-that the start times of the storage intervals of the respective channels of the CCD image sensor are made different in order to reduce the noise of the color boundary part, and that the attendant shift of the output signals is compensated by the buffer memory circuits disposed externally.
Since the prior-art system is constructed as described above, it requires the buffer memory circuits which realize the different start times of the storage intervals of the CCD image sensor for the purpose of reducing the noise of the color boundary part and which compensate the attendant shift of the output signals. Therefore, the prior art has had the problems that the circuit arrangement is complicated and that the scanner becomes expensive.